Plants are all-natural sources of all things good for us, right? It turns out some of our favorite plant-based flavorings may do more harm than good.

Scientists from Johns Hopkins Kimmel Cancer Center report in the journal Food and Chemical Toxicology that teas, coffees and “smoky flavoring” could be damaging our DNA at levels comparable to that caused by chemotherapy drugs.

The food chemistry and biology researchers tested the effects of some popular foods and food flavorings on cell cultures in the lab and discovered that a well-known repair gene called p53 that protects cells from becoming cancerous, was highly activated by compounds in black and green teas, coffee and liquid smoke flavoring, which is used to add smokey flavor to sausages and meat substitutes. The foods caused a 30 fold increase in p53 activity when they were added to the cells, which is comparable to the effect that the chemotherapy drug etoposide can have on the cancer-suppressing gene.

p53 is stimulated when DNA is damaged, and the gene triggers a series of responses that attempt to repair the affected DNA. The greater the damage to the DNA, the more p53 becomes activated, and researchers have come to view p53 levels as a marker for DNA in distress. To measure the p53 activity, the researchers tagged the gene in a bunch of human cells to a fluorescent marker that would glow when the gene was activated, and then added diluted amounts of the foods and flavorings. They let the cultures sit for 18 hours. Cultures with the black and green teas, coffee and liquid smoke all began to glow, indicating that p53 was hard at work doing damage control. Tests with other flavorings, including fish and oyster sauces, smoked paprika, wasabi powder and kim chee, didn’t activate p53 to the same levels.

It turns out that these foods and flavorings share in common some chemicals — pyrogallol and gallic acid — that the researchers believe are responsible for damaging the DNA and setting off p53. Pyrogallol is found in smoked foods as well as hair dye, tea, cigarette smoke, and coffee. Gallic acid is a type of pyrogallol and is primarily found in coffees and teas. It’s not clear how these agents act on DNA, but the harm is concerning enough to raise the alarm for p53 to swoop in and attempt to right the genetic wrongs.

Previous studies have documented similar DNA damage from liquid smoke on the stomach lining in rats, but whether it has the same effect on humans isn’t known. On human cells, at least, the effect was striking. “We found that liquid smoke, when diluted a thousand fold, was still as strong as the concentration of etoposide in a cancer patient being treated with etoposide. In fact, it works much the same way. Etoposide in cancer patients damages DNA, that’s how you get rid of the cancers, but it also has side effects,” says study author Dr. Scott Kern, the Kovler Professor of Oncology and Pathology at the Johns Hopkins University School of Medicine.

Why would plants harbor such potentially damaging agents? It’s possible they help to protect them, primarily from herbivores looking for their next meal. “Plants have been trying to keep animals from eating them for a long time. The plants make poisons, and animals develop defense mechanisms to take on the poisons. They have done this to such a great extent that some of these initial poisons can be considered nutrients and just food,” says Kern.

Which means that their ability to cause changes in DNA isn’t necessarily a cause for alarm. “When you find something damaging in food, you can’t overreact. You have to think, is this one we could be made to handle normally, or is this one that should worry us? In this report, we don’t know the answer to that question,” he says. Some of the aberrations caused by these plant-based chemicals may be ones that p53 is perfectly capable of fixing, for example, although more research is needed to determine how extensive the damage is, and what effect those aberrations may have on our health. “There’s no doubt our body tries to repair [the damage]. It might do a very good job of it. So if we found the signature was a really weak one, I would worry a lot less,” says Kern. “It means we can repair this damage really easily. If the signature, however, involves big deletions of DNA or some structural DNA lesions it leaves behind, then we could look for these calling cards in diseases [such as cancer].”

In the meantime, the researchers are not suggesting that people stop drinking tea or coffee or enjoying smoky-flavored foods. Still, it doesn’t hurt to be cautious. Kern, who enjoys cooking himself, has switched to using a smoky scotch to flavor some of his foods.